The use of recirculation of part of the exhaust gas to the intake of the engine in order to reduce NOx emissions has become a common procedure, as illustrated by way of example in patent U.S. Pat. No. 3,982,395, and it is generally necessary for engines working at low or medium load with a lean fuel mixture. In fact, introducing exhaust gases into the cylinders in the intake phase has the effect of decreasing the amount of fresh air that can mix with the fuel. This allows to lower the combustion temperature of the fuel mixture and consequently to reduce the production of NOx resulting from this combustion.
Generally, this recirculation is achieved by means of a circuit referred to as EGR (Exhaust Gas Recirculation) circuit, comprising a line connecting the exhaust manifold of the engine to its intake manifold and a valve, referred to as EGR valve, placed in this line and allowing to control the flow of gas circulating therein.
This circuit has the drawback of being very complex as regards installation of the line and of the valve, of leading to quite significant additional costs and of requiring very sophisticated EGR valve control means.
Even greater disadvantages add further to those described above in the configuration where supercharging of the engine is to be improved.
In fact, when the rotating speed of the turbine has to be rapidly increased, it is often accepted that, in the cylinders, a fuel mixture is achieved in stratified form and under lean-burn conditions so as to increase the amount of exhaust gas resulting from the combustion. Such a mixture leads to a combustion that produces a large amount of NOx and it is desirable to minimize this NOx production by exhaust gas recirculation. Furthermore, such a stratified fuel mixture also allows to significantly reduce the fuel consumption. However, during exhaust gas recirculation, pumping phenomena due to the pressure conditions required for this recirculation appear and these pumping losses greatly reduce the fuel consumption gains that could be expected from the stratified fuel mixture.
The present invention aims to overcome the aforementioned drawbacks by means of a method for controlling recirculation of exhaust gas allowing to do without a recirculation line and an EGR valve while using the necessary and sufficient amount of exhaust gas to allow the NOx emissions to be controlled. Furthermore, pumping losses are nearly eliminated and the fuel consumption reduction is increased.